Two methods for preparing ultrathin semipermeable membranes are described in U.S. Pat. No. 3,580,841--Cadotte et al (column 5, line 54--column 6, line 29). In the first method a concentrated solution of a polysaccharide polymer is poured upon a liquid surface and is permitted to desolvate leaving the polymer membrane as the residue. Membrane thickness (500 A-5000 A) is purportedly controlled by regulating the casting solution concentration and viscosity and, in some instances, by manually extending or expanding the solution. The second method consists of slowly drawing a clean glass plate from a dilute solution of a polymer in a solvent. It is indicated that the glass withdrawal rate may be used to control the thickness of the membrane.
In U.S. patent application Ser. No. 177,988--Lundstrom, filed Sept. 7, 1971 (now U.S. Pat. No. 3,767,737), ultrathin polymer membranes are prepared by continuously transferring a polymer-containing casting solution upwardly through a floating support liquid for deposition on the upper surface thereof. Some distance away from the station at which this deposition of casting solution occurs, a moving flexible inert support picks up solidified polymer film and pulls it along the support liquid away from the aforementioned deposition station. In this manner new casting surface is continually made available over which the casting solution can spread as it leaves the deposition station. After the casting solution has moved only a short distance from the deposition station most of the casting solution will have evaporated and a continuous polymer film is created. The Lundstrom patent is assigned to the assignee of the instant invention. The thickness of membrane prepared by this method is stated as being in the range of from about 0.005 mil (1300 A) to about 0.05 mil (13,000 A).
Another technique for casting a polymer film on a liquid casting substrate is described in the Pate and Yaffe article, "A New Material and Techniques for the Fabrication and Measurement of Very Thin Films for Use In 4.pi.--counting" [Canad.J. Chem. 33, 15 (1955)]. Films of polyvinychloride-acetate (PVCA) copolymer resin are prepared from a casting solution of PVCA in cyclohexanone. Because such a casting solution does not spread satisfactorily on a water surface (i.e. is not able to spontaneously spread over available water surface), conventional techniques for film formation could not be used. By the new method a trough was filled with water at room temperature and a floating wooden barrier was placed in contact with one end. A small amount of the resin solution was pipetted between the barrier and the trough wetting both the barrier and the trough. The barrier was released and the resin solution expanded to a band about 2-3 cm wide and as the edge nearest the barrier became solid the barrier was lifted from the water and lightly placed on the solidified edge of film. The barrier and the solidified edge were moved away over the water surface pulling behind them a film of resin observed to "feed out of the solution band". The resin film continued to be generated in this manner covering the water surface until either the barrier reached the end of the trough or the casting liquid was exhausted.
Thus, in order to carry out either of the latter processes (Lundstrom and Pate et al) the solid film has to be moved relative to the casting substrate. Such manipulative action inherently subjects the solid film to tensile stress. Therefore, in order to survive this action the solid film must be strong enough in tension either because of its thickness or because of the polymer resin employed. The art is still in need of a reliable reproducible method which obviates these limitations, while yielding ultrathin polymer film areas (greater than 1 square foot) amenable to the construction of commercially useful devices, e.g. for gas separation.